In a virtual disk library device access of data stored in a tape can be conducted at a high speed with a high-speed archiving process from a disk to the tape in units of LUs maintained. The efficiency of update of the data stored in the tape can be improved. In this system, when the data stored in the tape is to be accessed with a high-speed archiving process in units of LUs from the disk to the tape maintained, a disk staging process is performed in the unit of a page that is smaller than the unit of an LU, and accordingly, high-speed access can be implemented. In addition, when an update process for the data (LU) stored in the tape is performed, the host can be responded by only staging an update target page of the LU, and a prior disk staging process is performed for the remaining pages of the LU in the background. Accordingly, when the data is re-stored in the tape, immediately storing the data in the tape can be performed without performing a staging process.